A funny thing happened on the afternoon of the autumn statement. Barely had George Osborne sat down last week after announcing cuts in tax credit for children, and a giveaway to motorists, than Treasury civil servants began claiming that the richest 10% of Britons would in fact be hit hardest. Ministers were still at it at the weekend claiming the budget was "fair". Their proof? Why, none other than an official budget appendix called Impact on households: Distributional analysis to accompany the Autumn Statement 2011. Except this wasn't solely an account of winners and losers from the mini-budget – but also counted some measures passed by Gordon Brown. Focus on Mr Osborne's own initiatives, as the Institute for Fiscal Studies did the next day, and you ended up with a package that hit the poorest hardest and put money in the pockets of the richest.

When it comes to measuring fairness, the coalition likes pulling that here's-something-Labour-prepared-earlier trick. Last summer the chancellor claimed his so-called emergency budget hit the rich hardest. That time, too, the IFS had to point out that that was only because of policies created by Alistair Darling, which the new government had accepted. On new measures, that budget was judged as regressive (as was the autumn spending review: one does not need to be a fashion editor to spot a trend here). Later, Nick Clegg wrote a plaintive column for the FT about the definitions of the word fairness.

Fine, but the terms regressive and progressive are not so slippery: they are about whether poor people are left financially worse or better off. And he and David Cameron have made plenty of claims to being progressives. Besides, does Mr Cameron really want to take credit for not reversing decisions made by previous governments? Follow that logic and you end up in the land of the absurd, with prime ministers taking credit for not abolishing the NHS.

This may look like an issue to do with tables and charts, but at heart it is about the trust voters place in politicians. The chancellor claims to "fit the budget to fit the figures, instead of fixing the figures to fit the budget". To help him achieve that laudable goal, let us propose a small but significant tweak: the independent Office for Budget Responsibility should calculate who gains and who loses from each package of measures, and publish it on the same day. That should avoid political tricksiness and give voters an honest reading of the fairness of each budget. Not so long ago, Mr Osborne declared, "My politics are unapologetically progressive." Let's have an equally unapologetic gauge of that claim. It's only fair.